As others have alluded, you will likely damage or destroy your board and/or RAM if you put DDR1 into a DDR2 slot - which is why it is notched as such to prevent you from physically doing that.

However, if you simply must use that DDR1 ram, there are motherboards that have slots from both. The entry-level Intel 915 series, which as far as I know are not even manufactured anymore, sometimes have slots for both, as do budget boards from other various companies.